Fuel cell power systems convert a fuel and an oxidant to electricity. One fuel cell power system type of keen interest employs use of a proton exchange membrane or “PEM” to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air/oxygen) into electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system. The PEM has a reactive electrode disposed on each major face to form a membrane electrode assembly or MEA.
In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells have flow fields with inlets to fluid manifolds; which transport the various reactant feed streams in the stack to flow into each cell. Gas diffusion assemblies then provide a final fluid distribution to further disperse reactant feed stream from the flow fields to the reactive electrode of the MEAs.
Effective operation of a PEM requires adequate humidification of the PEM polymer to maintain its proton conductivity while maintaining transportation and distribution of the reactant feed streams in non-flooded operational states. In this regard, the oxidant, typically oxygen or oxygen-containing air, is supplied to the cathode where it reacts with hydrogen cations that have crossed the proton exchange membrane and electrons from an external circuit. Thus, the fuel cell generates both electricity and water through the electrochemical reaction. The water is removed with the cathode effluent, and, by some appropriate means of water vapor transfer, is used to humidify the inlet air stream. Without such humidification of the reactant streams, it is possible that under some conditions the cathode channels could evaporate water from the proton exchange membrane at an even higher rate than the rate of water generation (with commensurate dehydration of the PEM) via reaction at the cathode.
When hydrated, the polymeric proton exchange membrane possesses “acidic” properties that provide a medium for conducting protons from the anode to the cathode of the fuel cell. However, if the proton exchange membrane is not sufficiently hydrated, the “acidic” character diminishes, with commensurate diminishment of the desired electrochemical reaction of the cell.
A problem, however, in membrane hydration occurs when sufficient water is present in the two-phase flow of vaporized water and air to induce flooding in the reactant channels or the diffusion media of the cathode of the fuel cell, restricting reactant oxygen (i.e. oxygen in the air feed) from reaching catalytic sites at the membrane surface. Flooding typically occurs when the accumulation of liquid water is sufficient to adversely impact the flow of reactant gases through the flow channels or the diffusion media in a given cell or cells. Flooding degrades fuel cell performance because the accumulation of liquid water, either in the diffusion media or flow field channels, restricts access of reactant gas flows to catalytic sites of the membrane-electrode assembly (MEA) containing the PEM. Furthermore, insofar as flooding affects the temperature gradient in the plane of the MEA, flooding plausibly impacts durability of the fuel cell.
A partial solution to the flooding problem is to maintain a relatively “high” gas velocity in the flow channels distributing air (oxygen) for the cathode so that the water remains entrained in the cathode effluent. Another solution is to terminate or restrict the rate of water supplementation when flooding is detected; however, this is inherently a remedial action to minimize damage rather than a proactive approach to prevent damage insofar as a determination that flooding has occurred inherently means that some damage from flooding did occur within the fuel cell.
What is needed is a fuel cell power system providing (a) full humidification of the feed gases (especially the oxidant), (b) an accurate determination of the onset of flooding status, and (c) control action responsive to the determination of flooding onset sufficient to preclude flooding from actually occurring within the fuel cell. The present invention is directed to fulfilling this set of needs.